Boolean modifiers do not work, when boolean objects are in certain positions
Boolean doesn't work when faces are overlapping, at least until the recent versions, maybe they will fix that, so you need to find another method for the union one, maybe simply do it manually.



About the Bevel modifier, you've choosen None mode, so it will bevel every edge, and you've enabled Clamp Overlap option, that won't allow you to do any overlapping. With the given topology you won't be able to bevel anything without overlapping. So what you should choose is the Angle mode, or perhaps Weight if you want various bevel strengths depending on the edges, and disable Clamp Overlap. But even with those options I'm not sure it will be easy to manage your topology.